These are the ISO files for the NeXT Computers OS, NeXTStep 3.3.
The user disc contains all the OS installation stuff, and the Developers disc contains all the stuff you need to compile programs, which is a must for this system.
Since to my knowledge, no NeXT computer ever came with a CD-ROM Drive built in, you must use an external one. Be careful though, some older CD-ROM drives do not work with CD-R's!